Extending structures, Galois groups and supersolvable associative algebras

Abstract

Let A be a unital associative algebra over a field k. All unital associative algebras containing A as a subalgebra of a given codimension \(\mathfrak {c}\) are described and classified. For a fixed vector space V of dimension \(\mathfrak {c}\), two non-abelian cohomological type objects are explicitly constructed: \({\mathcal {A}}{{\mathcal {H}}}^{2}_{A} \, (V, \, A)\) will classify all such algebras up to an isomorphism that stabilizes A while \({\mathcal {A}}{{\mathcal {H}}}^{2} \, (V, \, A)\) provides the classification from Holder’s extension problem viewpoint. A new product, called the unified product, is introduced as a tool of our approach. The classical crossed product or the twisted tensor product of algebras are special cases of the unified product. Two main applications are given: the Galois group \(\mathrm{Gal} \, (B/A)\) of an extension \(A \subseteq B\) of associative algebras is explicitly described as a subgroup of a semidirect product of groups \(\mathrm{GL}_k (V) \rtimes \mathrm{Hom}_k (V, \, A)\), where the vector space V is a complement of A in B. The second application refers to supersolvable algebras introduced as the associative algebra counterpart of supersolvable Lie algebras. Several explicit examples are given for supersolvable algebras over an arbitrary base field, including those of characteristic two whose difficulty is illustrated.